The stepson of murdered woman Karen Ristevski has backed a key claim about the day she died, in a new twist in the murder investigation.

The 47-year-old mother's body was found in February, 2017 in bushland in Mt Macedon, about an hour’s drive northwest of the city after vanishing on June 29, 2016.

Her husband, Borce, who has denied being involved in his wife's death, claimed after her Mercedes-Benz was spotted driving in Mt Macedon when she disappeared, that he had taken the car for a test drive to test a faulty fuel gauge.

CCTV footage revealed a black Mercedes identical to Ms Ristevski's being driven into her street on the day she disappeared. Source: 7 News

Mr Ristevski's estranged son Anthony Rickard this week told the Herald Sun he supported his father's claim the fuel gauge on the woman's car was broken. However, he said it had been faulty since 2014 and his dad was too cheap to have it fixed.

Mr Ristevski previously told police he took the SLK 200 for a test drive on the Calder Freeway on June 29 until a bump in the road caused the fault to suddenly correct itself.

Last month, a Keilor East mechanic, who worked on the car, reportedly told detectives in all his time servicing the vehicle, neither Mr Ristevski nor his wife had ever asked him “to fix any faulty fuel gauge”.

There is a huge significance being placed on the black Mercedes in the investigation after it was captured on CCTV being driven through Diggers Rest, towards Mt Macedon on the same day Mrs Ristevski disappeared.

Karen Ristevski disappeared in June 2016. Source: 7 News

Last month police used a black Mercedes to recreate the journey from their Melbourne home through Diggers Rest, towards Mt Macedon, to the bushland location her body was found.